There is a light-emitting apparatus which applies a light-emitting diode (LED) chip as a semiconductor light-emitting element for a light source. Since an emitting efficiency of the LED is improved, the LED is employed as a light source of a relatively large luminaire in an office and for general-purpose illumination. To increase an application of the LED as the light source, an LED having a high conversion efficiency and a high output is required. To realize the LED having a high output, it is important how to efficiently radiate the heat generated when the LED emits light.
There is a light-emitting apparatus which lights an LED chip by a power of 1 watt [W] or more. A substrate, which is formed with a first metal layer, an insulation core, and a second metal layer, is used as a substrate on which an LED chip is mounted. The insulation core is made of flat ceramics. The first metal layer is made of copper or aluminum excellent in, a conductive property to wire the LED chip, and forms a pattern on one surface of the insulation core. LED chips are mounted on the patterned first metal layer. The second metal layer is made of copper or aluminum excellent in thermal conductivity to transfer heat by being connected to a radiation component without bearing an electric connection function, and approximately integrally and flatly formed on a surface of the insulation core on a side opposite to the first metal layer. Since the light-emitting apparatus uses the material excellent in thermal conductivity as the second metal layer, the light-Omitting apparatus efficiently diffuses the heat generated by the LED in the substrate. Therefore, the light-emitting apparatus operates the LED by a large current in comparison with an apparatus which has no radiation function.
The first metal layer and the second metal layer are directly bonded or brazed, respectively. A substrate, which is configured by directly bonding the first metal layer and the second metal layer to the insulation core, is called a direct copper bonding (DCB) substrate. A substrate, which is configured by bonding the first metal layer and the second metal layer to the insulation core with lead-free solder, is called an active metal bonding (AMB) substrate. Since the insulation core and the first metal layer or the second metal layer, which configures the DCB substrate and the AMB substrate, are made of a different material, the insulation core and the first metal layer or the second metal layer naturally have a different thermal expansion rate and a different heat transfer speed. The first metal layer is a metal layer partly formed on the insulation core to form a wiring pattern. The second metal layer is a metal layer formed on an approximately overall surface thereof to exhibit a radiation operation. As described above, since the first metal layer and the second metal layer have a different pattern, the substrate is outstandingly warped in its entirety, when the first metal layer and the second metal layer are exposed to a high temperature atmosphere or an abrupt temperature change in a manufacturing stage.
A handling property is deteriorated in a process of manufacturing the light-emitting apparatus when the substrate is warped in its entirety. Further, a light-emitting apparatus, which is completed in a state that it is warped, cannot sufficiently obtain an area in contact with a case component for radiation and a radiation component such as a heat sink when the light-emitting apparatus is assembled to a luminaire. That is, the light-emitting apparatus cannot transfer the heat generated by an LED chip to the case component or the radiation component. Therefore, a light emitting efficiency is lowered and a luminous flux having a designed value cannot be obtained. Further, gaps are formed between the substrate and the case component or the radiation component when the substrate is warped. When the substrate is tightened more than necessary with a fastener such as a screw to eliminate the gaps, the substrate may be broken from a crack generated therein.
In the DCB substrate, a thickness of the first metal layer is increased to enhance a heat resistance. The second metal layer of the DCB substrate is thinner than the insulation core and bonded to a heat sink via a connection material such as solder, heat grease, phase change tape, or heat blanket. Alternatively, the DCB substrate includes a base which is thicker and wider than the second metal layer and connected to the second metal layer. The base is fastened to a heat sink and the like by bolts or rivets which are caused to pass through two cutouts formed in a peripheral portion. However, the number of parts increases and a cost becomes high, when the base is provided separately from the DCB substrate.
Solder has a melting temperature of about 230° C. even if the solder is lead-free solder, and the melting temperature is lower than a heat resistant temperature of an adhesive used to mounts an LED chip on the first metal layer. The solder may be deteriorated or cracked by high temperature creep and thermal stress fatigue, when a light-emitting apparatus is used under a high temperature environment or repeatedly subjected to a temperature change by being turned on and off, in the case that the second metal layer is attached to a heat sink using solder as a connection material. A power supplied to the LED chip needs to be suppressed so that a temperature of the DCB substrate does not become excessively high to keep a solder portion in a good state. However, this contradicts an increase in brightness of the LED chip.
As a method of directly fastening a heat sink to the DCB substrate, it is considered to fasten the heat sink to the DCB substrate by using a screw hole formed in an insulation core of the DCB substrate. However, the method is not preferable in two points described below. First, the insulation core may be broken, when the torque that tightens a screw is excessively strong, because the insulation core is made of ceramics. Second, since a second metal layer of the DCB substrate and the heat sink have a coefficient of linear expansion larger than that of the DCB substrate, the DCB substrate may be warped, when temperatures of the DCB substrate and the heat sink increase. In this case, the insulation core may be broken by stress concentrated on the screw hole. Accordingly, a technology is required that fastens the DCB substrate to a radiator such as the heat sink without using a base as a part separate from the light-emitting apparatus.
Further, there is a light-emitting apparatus that uses ceramics having a high thermal conductivity as a circuit board on which an LED is mounted. Generation of distortion in the circuit board is prevented by transferring the heat generated by the LED through the circuit board. The light-emitting apparatus has a metal attaching pattern formed on a side opposite to a side where the LED is mounted. The attaching pattern of the light-emitting apparatus is bonded to a radiation board opposed against the attaching pattern with lead-free solder. The radiation board is formed of the same material as the attaching pattern or a material having approximately the same thermal expansion rate as the attaching pattern to prevent generation of a crack when the attaching pattern is bonded to the radiation board with solder. The light-emitting apparatus includes an open region around a main bonding portion formed between the attaching pattern corresponding to a location where the LED is arranged and the radiation board. The open region prevents a crack from reaching the main bonding portion. However, in this light-emitting apparatus, prevention of overall warping of the substrate is not taken into consideration.